University of California Institute of Transportation Studies

Traffic-related emissions are divided into two general categories: exhaust- and non-exhaust-related. Due to decades of efforts to reduce exhaust-related emissions, the relative contribution from non-exhaust sources has increased. In contrast to exhaust-related emissions, non-exhaust sources are not well studied and their characteristics such as emission factors and associated health effects need more investigation. Previous studies have shown that the contribution from non-exhaust sources to PM10 (particulate matter with an aerodynamic diameter equal to or less than 10 micrometers) which can cause severe respiratory problems, is approximately equal to exhaust-related sources. The major contributors to non-exhaust particulate matter are brake and tire wear, while minor contributors include clutch and engine particle emissions.